Vasoconstrictor fibers of the sympathetic nervous system supply the arteries, arterioles, and veins; the neural influence is much less on larger vessels than on arterioles and small arteries. Capacitance vessels (veins) respond more to sympathetic nerve stimulation than do resistance vessels; the capacitance vessels are maximally constricted at a lower stimulation frequency than the are resistance vessels. However, capacitance vessels lack β-adrenergic receptors, and they respond less to vasodilator metabolites. Norepinephrine is the neurotransmitter released at the sympathetic nerve terminals in blood vessel. Factors such as circulating hormones and particularly locally released substances mediate the release of norepinephrine from the nerve terminals.
The response of the resistance and capacitance vessels to stimulation of sympathetic fibers is illustrated in
Fig.
18.24 . When arterial pressure is held constant, stimulation of sympathetic fibers reduces blood flow (constriction of resistance vessels) and decreases the blood volume of the tissue (constriction of capacitance vessels). Constriction of the resistance vessels establishes a new equilibrium of the forces responsible for filtration and absorption across the capillary wall (see
Eq. 17.20
In addition to active changes (contraction and relaxation of vascular smooth muscle) in vessel caliber, passive changes are also caused by alterations in intraluminal pressure. An increase in intraluminal pressure distends the vessels, and a decrease reduces the caliber of the vessels as a consequence of elastic recoil of the vessel walls.
At basal vascular tone, approximately a third of the blood volume of a tissue can be mobilized when the sympathetic nerves are stimulated at physiological frequencies. Basal tone is very low in capacitance vessels; if these vessels are denervated experimentally, the increases in volume evoked
Change of tissue Arterial pressureBlood flow volume (mL) (mm Hg) •Fig. 18.24 Effect of Sympathetic Nerve Stimulation (2 Hz) on Blood Flow and Tissue Volume of the Lower Limb. Theupward arrow denotesthechangeinslopeofthetissuevolumecurveatthepointatwhichthedecreaseinvolumecausedbyemptyingofcapacitancevesselsceasesandlossofextravascularfluidbecomesevident.Theabruptdecreaseintissuevolumeiscausedbymovementofbloodoutofthecapacitancevesselsandoutofthelowerlimb.Thelate,slow,progressivedeclineinvolume(totherightofthearrow)iscausedbythemovementofextravascularfluidintothecapillariesandhenceawayfromthetissue.Thelossoftissuefluidresultsfromtheloweringofthecapillaryhydrostaticpressuresecondarytoconstrictionoftheresistancevessels.(FromMellanderS.Acta Physiol Scand Suppl. 1960;50[176]:1.) by maximal doses of ACh are small. Therefore, at basal vascular tone, blood volume is close to the maximal blood volume of the tissue. More blood can be mobilized from the capacitance vessels in the skin than from those in the muscle. This disparity depends in part on the greater sensitivity of the skin vessels to sympathetic stimulation, but it also occurs because basal tone is lower in skin vessels than in muscle vessels. Therefore, in the absence of a neural influence, skin capacitance vessels contain more blood than do muscle capacitance vessels.
Physiological stimuli mobilize blood from capacitance vessels. For example, during physical exercise, activation of sympathetic nerve fibers constricts the peripheral veins and hence augments cardiac filling pressure. In arterial hypo-tension (as occurs in hemorrhage), the capacitance vessels constrict and thereby correct the decreased central venous pressure associated with blood loss.
The efferent fibers of the cranial division of the parasympathetic nervous system innervate the blood vessels of the head and some of the viscera, whereas fibers of the sacral division innervate blood vessels of the genitalia, bladder,